The Dora-Maira Massif

Shaped as an “ellipsoid” of about 70 × 25 km, the Dora-Maira Massif is the crystalline basement of the Cottian Alps. It consists of an upper complex formed of pre-Carboniferous metasediments and metabasites, and a lower complex composed of (Permo)-Carboniferous metasediments. Both complexes contain meta-intrusives of granitic to dioritic composition, which are mainly regarded as Variscan. The sedimentary protoliths in the pre-Carboniferous basement are represented mostly by pelites with subordinate, more or less dolomitic limestones, whereas coarser clastic facies predominate in the (Permo)-Carboniferous cover. The Mesozoic sequence overlying the massif is similar to that of the other Penninic units.The Alpine, tectono-metamorphic overprint is very strong throughout the massif: disregarding the “Polymetamorphic Unit” with its very high-pressure assemblages, the conditions of the early-Alpine metamorphic event in the massif are comparable with those of the other internal units of the Western Alps. The eclogite and blueschist facies stages were followed — and in some cases almost completely obliterated — by recrystallization into the greenschist facies. The Alpine metamorphism was accompanied by tectonic events including at least three phases of ductile deformation.